In general, in order to form a desired image on an image-carrying medium such as paper, the desired image is temporarily duplicated on an image-forming surface 5 before transferring it onto the image-carrying medium. More particularly, referring to FIG. 1, a conventional image-forming system is illustrated. A light source 1 emits a light beam through a colimeter lens 2 towards a rotatable scanner 3 such as a polygon mirror having multiple reflecting surfaces. The beam is reflected by one of the reflecting surfaces towards an image-forming surface 5 such as a photoreceptor drum via a f.theta. lens 4. Since the scanner 3 is rotated as indicated by an arrow, the reflected beam is scanned over the image-forming surface 5 in a predetermined scanning direction as indicated by another arrow. Consequently, a desired image is formed by the scanning beam as the photoreceptor drum rotates in a sub-scanning direction as indicated by a third arrow.
Still referring to FIG. 1, to scan the beam in order to form a desired image, a precise onset timing is required for each scanning beam to initiate the image formation at a predetermined location on the image-forming surface 5. To accomplish the above described precise timing, one method is to detect the scanning beam prior to reaching the image-forming surface 5 at a predetermined beam detection location outside the image-forming surface 5 and then to delay the initiation of the image forming process by a predetermined amount of time. To implement the above method, a beam detector 6 such as a photo sensor is located near the photoreceptor drum 5, and after the scanning beam is detected, the image forming process by the same scanning beam is delayed by a predetermined amount of time based upon a constant predetermined rotational speed of the scanner 3. The above implementation also requires that mechanical parts are not thermally effected to cause a substantial variation in the desired predetermined delay. However, neither the constant rotational speed nor the thermal expansion effect is generally guaranteed in an image forming apparatus.
Referring to FIG. 2, to improve the above described problems, Japanese Patent Hei 5-60085 which was published on Sep. 1, 1993 discloses a single beam synchronization confirmation system for synchronizing each sweep of a single scanning beam to initiate the image formation at a predetermined location on an image-forming surface. The single beam synchronization confirmation system includes a synchronization signal modulation unit 11 for generating a synchronization modulation signal indicative of an expected arrival timing of the scanning beam at a photosensor and a photosensor for detecting the scanning beam and generating a photosensor output signal. The synchronization modulation signal is logically ANDed with the photosensor output signal at an AND gate 16. The AND gate outputs a high signal or a synchronization signal only when the synchronization modulation signal and the photo sensor output signal are contemporaneous. The synchronization signal is fed back to the synchronization signal modulation unit 11 for further modulating the synchronization modulation signal as well as to an image scanning clock signal generation unit 12 for generating a scanning clock signal. Based upon the scanning clock signal and the synchronization modulation signal, an image control unit 13 initiates an image generation unit 14 and drives a light source activation unit 15 for activating the beam according to a desirable image.
Now referring to FIG. 3, the above described single beam synchronization confirmation system performs the synchronization as illustrated in a timing chart. The synchronization modulation signal has a period T.sub.0 and an image forming period of T. In other words, a scanning beam has a period T for forming an image along one line on an image forming surface at a frequency of 1/T.sub.0. The synchronization modulation signal remains high for a predetermined amount of time. During this onset period of the synchronization modulation signal, a scanning light beam is expected to arrive at a photosensor. When the photosensor indeed detects the expected scanning beam, the photosensor generates a photo sensor output signal. As described above, a logical AND gate compares the photo sensor output signal and the synchronization modulation signal, and its HIGH output is a synchronization signal. In response to the synchronization signal, the synchronization modulation signal is deactivated until a next onset after an amount of time T.sub.0.
Still referring to FIG. 3, based upon the synchronization signal, after the predetermined delay following the synchronization signal, an image signal is initiated so as to form a desired image at a predetermined location on an image forming surface. A modulation signal A illustrates a positive-positive process with erasing signals juxtaposing the image signal while a modulation signal B illustrates a negative-positive process without the erasing signals. In contrast, in the absence of the synchronization modulation signal, an accidental photo sensor output signal such as a noise A alone does not cause the AND gate to generate the synchronization signal. By the same mechanism, when the light beam detection does match the expected arrival indicated by the synchronization modulation signal, the synchronization signal is not generated, and the image formation is prevented from starting at an undesirable location on an image forming surface. Because of the above described safeguarded generation of the synchronization signal, the onset for the image formation is more precisely determined.
In applying the above described single beam confirmation technique to a multiple beam image forming process, referring to FIGS. 4, a synchronization signal modulation unit 20 generates a synchronization modulation signal SMS and is connected to an image generation control unit 32 for forming a desired image using multiple light beams Ld.sub.1 through Ld.sub.n based upon the common synchronization modulation signal. During an image formation, an image data output unit 30 outputs an image output signal to the image generation control unit 32 in order to form a desired image. An AND gate 24 also receives the synchronization modulation signal as well as a photosensor detection signal from a photosensor 22 which is located at a predetermined light detection location outside of an image forming surface. If the above two signals are simultaneously activated, the AND gate outputs a HIGH or ON signal to a synchronization signal generation unit 26. Based upon the AND gate ON signals, the synchronization signal generation unit 26 sequentially activates a series of synchronization signals SS.sub.1 through SS.sub.n each of which is supposedly indicative of a corresponding light beam arriving at the photosensor 22. Upon the activation of the first synchronization signal SS.sub.1, a line counter 28 is initialized, and upon completing a predetermined amount of time, the line counter 28 outputs a line completion signal to the synchronization signal modulation unit 20, and the synchronization signal modulation unit 20 activates a synchronization modulation signal SMS.
Referring to FIGS. 5A and 5B, timing charts illustrate a synchronization problem associated with the above described multiple beam image forming system. FIG. 5A illustrates a situation where every one of the multiple beams Ld.sub.1 through Ld.sub.n is correctly activated, scanned by a scanner and sequentially detected by a common photo sensor. For the purpose of simplicity, only the beams Ld.sub.1 and Ld.sub.n are represented in solid lines in the timing chart, and any other beams between the two beams are abbreviated and illustrated by dotted-line representations. In response to an onset of a line counter signal LCS, a synchronization modulation signal SMS is activated. During the activated synchronization modulation signal SMS, the multiple beams are expected to sequentially arrive at the common photosensor. For each detection of the multiple beams Ld.sub.1 through Ld.sub.n, a corresponding photosensor detection signal (PSDS.sub.1 through PSDS.sub.n) is generated. Each of the photosensor detection signals PSDS.sub.1 through PSDS.sub.n and the synchronization modulation signal SMS are logically ANDed to generate a corresponding synchronization signal (SS.sub.1 through SS.sub.n). Upon the detection of a predetermined n number of photosensor detection signals, the synchronization modulation signal SMS is deactivated until the next line counter signal.
Now referring to FIG. 5B, for various reasons, every one of the multiple beams Ld.sub.1 through Ld.sub.n is not always correctly activated, scanned or detected. As described above, a timing chart illustrates that a synchronization modulation signal SMS is activated in response to a line completion signal LCS. This timing chart also illustrates a situation where a first photosensor detection signal PSDS.sub.1 is not generated, and the missing first photosensor detection signal PSDS.sub.1 is indicated by dotted lines. For example, this failure may be caused by an obstructed beam path, a malfunctioning photosensor or a malfunctioning light source. Because of this missing photosensor detection signal, a corresponding synchronization signal SS.sub.1 is not generated. For the rest of the photosensor detection signals PSDS.sub.2 through PSDS.sub.n, although corresponding synchronization signals SS.sub.2 through SS.sub.n are generated, the first synchronization signal SS.sub.2 is counted as a first synchronization signal, and thus, a number of actually generated synchronization signals becomes n-1 after the PSDS.sub.n. In order to complete the count, the first synchronization signal SS.sub.2 should be treated as the second synchronization signal since another synchronization signal should have preceded the first synchronization signal SS.sub.2. Consequently, the synchronization modulation signal is left activated after the last photosensor signal PSDS.sub.n due to the failure to count the predetermined n synchronization signals.
Other problems associated with synchronizing multiple beams include off-timing of multiple beams arriving at a photosensor. In other words, even if a photosensor detects each of the multiple beams, some of the beams may not be traveling at an assumed velocity and arrive at the photosensor in an unexpected time frame. For example, this may be caused by an unstable rotational speed of a polygon mirror. Under such a condition, the above described prior art technique does not allow each individual beam arrival to generating a corresponding accurate synchronization signal for an initiating image writing process.
Additional prior art attempts for synchronizing multiple beams include Japanese Patent Hei 6-246964, which discloses a synchronization circuit to trigger an independent multivibrator directly in response to a photo sensor output signal indicative of detecting each beam. An output signal from the multivibrator is used to synchronize an image processing process. This and other prior art attempts such as Japanese Patents Hei 2-188731 and Hei 3-76063 also disclose slits placed between a light source and a photosensor for improving the photosensor detection of each of the multiple beams.
As described above, the application of the prior art synchronization techniques does not solve a number of problems associated with multiple beam synchronization. In particular, the prior art techniques fail to identify or ascertain individual beams for the photosensor detection as well as the generation of a synchronization signal. The current invention is directed to ascertain the beam activation, the beam detection as well as the synchronization signal generation for each of multiple beams.